In the prior art (U.S. Pat. No. 6,179,132), porous membranes for filtration, and not for fuel cells, are described; they comprise a porous perfluoropolymer substratum having the surface completely modified by a perfluorocarbon copolymer directly bound to the substratum, the perfluorocarbon copolymer having hydrophilic groups directly wettable at contact with water. In said patent it is stated that the perfluoropolymer surface is rendered hydrophilic without compromising the substratum inertia and without meaningfully decreasing the substratum porosity. The copolymer is deposited on the perfluoropolymer from a substantially aqueous solution to obtain a perfluoropolymer surface directly wettable with water. This directly wettable surface modified according to the process described in said patent differs from the surfaces described in the prior art modified with perfluorocarbon polymers, deposited from a solution of water and an organic solvent or of an organic solvent alone, since the surfaces are not directly wettable at contact with water. Besides the described surfaces modified according to the prior art require a complex pretreatment (organic solvent or shear) to allow the surface wetting with water. The porous membranes of said patent do not show the dewetting phenomenon. The support surface according to said patent is not coated by a coating but it is only modified. Said membranes show water permeability, but they cannot be used in fuel cells since tests carried out by the Applicant show that they do not show substantial ionic conductivity.
The proton exchange membranes, for example those for fuel cells, should show a high proton exchange capability (conductivity) combined with a high water permeability. The membranes at present used are Nafion® based and show a good conductivity, sufficient for the use in stationary plants, but not deemed suitable for the car field. Besides the water permeability is substantially absent. To improve the proton transport and therefore the membrane conductivity, membranes having a reduced thickness are used. However these thicknesses cannot be lower than about 100 microns in order not to jeopardize the mechanical stability of the membrane. Furthermore it is to be noted that the water permeability also of these membranes is very low. To further reduce the membrane thickness, composite membranes are known wherein an ionomer is deposited on a support which guarantees the mechanical stability thereof. It is thus possible to obtain thicknesses lower than 100 microns. For example membranes having a thickness lower than 50 microns have been obtained by using as a support bistretched PTFE having a high porosity. However these membranes have the drawback to have also in this case a water permeability substantially absent.
All the membranes for fuel cells of the prior art or available on the market, show a permeability to gases substantially absent (Gurley number >10,000). Besides these membranes once dehydrated are regenerable with difficulty, especially when the thicknesses are high. This is up to now an unsolved aspect which makes it difficult the fuel cells functioning.
Besides the fuel cells of the prior art use very pure hydrogen to have poisonings of the platinum-based electrodes. In fact if hydrogen from reforming, therefore containing CO, is used, a quick platinum poisoning takes place. According to the prior art the hydrogen from reforming must be purified from CO before being used in fuel cells.
The need was felt to have available hydrophilic membranes which with respect to the membranes for fuel cells of the prior art showed the following combination of properties:                improved water permeability;        absence of the phenomenon of difficult regeneration after dehydration;        a controllable porosity to gases;        a high conductivity in the cells;        possibility to operate also with hydrogen from reforming (containing CO) having a higher electrode life.        